505 GTD Estate Auto ( sensor question)

Hi, had my Pug for 2 years now, and had a Tacho problem on it for a while (it used to be intermittent, now is just DEAD) had alternator replaced as that was dead as well, and thought that would cure the Tacho, but it hasnt, recently i was fiddling around under the car and noticed a sensor wire coming out of the engine side of the auto transmission, ita aa big bulbous module with a blue wire bolted on to it, i traced the wire to find it changes to a white wire that disapears inside a python of wiring. Does anyone have any ideas how to fix the Tacho? Is that the actual wire/sensor for it? if not what the heck is it ???? Thankyou :)